Reports about diurnal changes of assimilates in phloem sap are controversial. We determined the diurnal changes of sucrose and amino acid concentrations and fluxes in exudates from cut aphid stylets on tansy leaves (Tanacetum vulgare), and sucrose, amino acid and K(+) concentrations and fluxes in bleeding sap of castor bean pedicel (Ricinus communis). Approximately half of the tansy sieve tubes exhibited a diurnal cycle of sucrose concentrations and fluxes in phloem sap. Data from many tansy plants indicated an increased sucrose flux in the phloem during daytime in case of low N-nutrition, not at high N-nutrition. The sucrose concentration in phloem sap of young Ricinus plants changed marginally between day and night, whereas the sucrose flux increased 1.5-fold during daytime (but not in old Ricinus plants). The amino acid concentrations and fluxes in tansy sieve tubes exhibited a similar diurnal cycle as the sucrose concentrations and fluxes, including their dependence on N-nutrition. The amino acid fluxes, but not the concentrations, in phloem sap of Ricinus were higher at daytime. The sucrose/amino acid ratio showed no diurnal cycle neither in tansy nor in Ricinus. The K(+)-concentrations in phloem sap of Ricinus, but not the K(+) fluxes, decreased slightly during daytime and the sucrose/K(+)-ratio increased. In conclusion, a diurnal cycle was observed in sucrose, amino acid and K(+) fluxes, but not necessarily in concentrations of these assimilates. Because of the large variations between different sieve tubes and different plants, the nutrient delivery to sink tissues is not homeostatic over time.